1. Field of the Invention
The present invention relates to a metal gasket made of a thin metal plate, which has a main seal portion formed with a hole such as a combustion chamber hole and a sub-seal portion formed with an opening such as a chain passage opening, and which is intended to seal an interface between a cylinder block with a chain case and a cylinder head.
2. Description of the Prior Art
The metal gaskets have been used to seal hermetically between a top surface of an integrally formed cylinder-block-and-chain-case assembly and a bottom surface of a cylinder head and comprise a main seal portion requiring a high sealing surface pressure and a sub-seal portion requiring only a relatively low sealing surface pressure. The sub-seal portion has a laminated structure having a base member, which is formed by extending at least one of the thin metal plates forming the main seal portion, and soft sheets, such as a beater sheet, a graphite sheet and a compressed sheet, bonded to one or both surfaces of the base member (see Japanese Utility Model Laid-Open No. 48059/1988, for example).
Other known examples of the metal gasket include those of Japanese Utility Model Laid-Open No. 27857/1991 and 27858/1991. In the metal gasket disclosed in Japanese Utility Model Laid-Open No. 27857/1991, a part of the thin metal plate facing a stepped portion of the top surface of the cylinder block or of the bottom surface of the cylinder head is formed with slots that cross beads formed around the chain passage opening, and a sealing material is fitted in the slots at positions opposing the stepped portions so that a part of the sealing material projects from the slots.
With the OHC and DOHC type engines, however, as shown in FIG. 17, it is general practice to fabricate a chain case 29 accommodating a cam shaft drive chain separately from a cylinder block 28 and to fix by bolts the chain case 29-which has a chain case space, i.e. a chain passage opening 33-adjacent to the cylinder block 28 having a combustion chamber 35. Metal gaskets are interposed between the cylinder head and the chain case 29 and cylinder block 28, with the chain case 29 secured to the cylinder head by bolts inserted through bolt holes 32.
For a lighter engine, the mounting side of the chain case 29 is formed open to reduce the weight of the chain case 29 and the chain passage opening 33 is defined by the inner wall of the chain case 29 and the side wall of the cylinder block 28. Because separate components are combined and fixed, even when the upper surface 34 of the cylinder block 28 and the upper surface 31 of the chain case 29 are made flush, it is unavoidable that a stepped portion 30, though small, is formed between these upper surfaces 31 and 34 at the engagement surface 36 of the cylinder block 28 and the chain case 29. This stepped portion 30 will unavoidably increase over a long period of use of the engine. If there is a stepped portion 30 between the two upper surfaces 31 and 34, lubricant, splashed onto the stepped portion 30 as the cam shaft drive chain travels through the chain case 29, may leak outside the chain case 29 or enter between the opposing surfaces of the cylinder block 28 and the cylinder head, causing troubles.
In a structure, in which a thin metal plate laminated on the cylinder block 28 side is extended as a base member, to one surface of which a soft sheet is bonded, airtightness at the stepped portion 30 cannot be maintained. Further, in a structure in which an intermediate thin metal plate is extended as a base member, to both surfaces of which soft sheets are bonded, although the airtightness can be maintained when the stepped portion 30 is very small, it is necessary, as the stepped portion 30 increases, to consider the thickness of the soft sheet in the main seal portion in designing the metal plate lamination because the soft sheet itself must be thick enough to maintain the sealing performance The beater sheet, graphite sheet or compressed sheet that forms a soft sheet generally has a thickness of at least about 0.5 mm, and because the main seal portion needs to match this thickness, it is necessary to increase the number of laminated thin metal plates to secure a high surface pressure at the main seal portion.
Further, the metal gasket has slots formed along the circumference of a through-hole, and a sealing material is installed inside the slots so that a part of the sealing material projects from the slots. The sealing capability of portions around the through-hole that oppose the stepped portions is maintained by the seal member installed inside the slots. The sealing material extends along the entire circumference of the through hole and is not fitted into the intermediate sheet.
Generally there are two types of stepped portions formed at the engagement surface between the cylinder block and the chain case. When the upper surface of the chain case is lower than the upper surface of the cylinder block, the surface pressure of the chain case is lower than that of the cylinder block and it is necessary to consider the surface pressure of the stepped portion and the chain case. When the upper surface of the chain case is higher than the upper surface of the cylinder block, the surface pressure concentrates on the chain case making the surface pressure on the cylinder block side smaller. The surface pressure around the combustion chamber of the cylinder block needs to be set high but, when a metal gasket is used, adjustment of the surface pressure becomes difficult.
The undersurface of the cylinder head in general is a flat continuous surface whereas there may be a difference in height between the top surface of the cylinder block and the top surface of the chain case. Thus the metal gasket has a challenging task of preventing oil in the chain case from entering into a stepped portion formed when there is a difference in height between the top surfaces of the cylinder block and the chain case, without resorting to increasing the number of laminated thin metal plates, a means that will increase the manufacture cost of the gasket significantly.
The inventor of this invention developed a metal gasket disclosed in Japanese Patent Laid-Open No. 74659/1996, in which a seal member is arranged in a region of stepped portion. With this metal gasket, however, when arranging the seal member in the region of stepped portion, there is still an unsolved problem of how to fix the seal member to the metal gasket at a predetermined location stably and firmly so that it will not be dislocated.